We propose to study several more instrumented conscious horses. The recording of left ventricular internal diameter, left ventricular pressure and aortic pressures will enable us to evaluate continuous ventricular volume, dimensional measurements and ventricular contractile capacity, myocardial contractility and volume-tension relationships. In addition we propose to study flow and pressures from the left main and circumflex coronary arteries by implantation of pressure cells and flow probes in and around these vessels. The effect of various interventions on the coronary flow and pressures will be studied and related to ventricular mechanics.